. Elementary physical geography;. y, Cascade, and Sierra Nevada Mountainsavalanches are frequent, but they are not so common as in the Alps. In the Alps, where the slopes are steep, avalanches occurfrequently and regularly. In many places the avalanche tracks are as definitelymarked as river chan-nels. Indeed, one mayconsider an avalanchetrack as the torrentialpart of a stream whoseflow is occasional andspasmodic. Like themountain torrent, too,it carries to lower lev-els an enormous amountof rock waste. Not onlyare avalanche coursesdistinctly marked, butexpert mountaineers areable to predict t
. Elementary physical geography;. y, Cascade, and Sierra Nevada Mountainsavalanches are frequent, but they are not so common as in the Alps. In the Alps, where the slopes are steep, avalanches occurfrequently and regularly. In many places the avalanche tracks are as definitelymarked as river chan-nels. Indeed, one mayconsider an avalanchetrack as the torrentialpart of a stream whoseflow is occasional andspasmodic. Like themountain torrent, too,it carries to lower lev-els an enormous amountof rock waste. Not onlyare avalanche coursesdistinctly marked, butexpert mountaineers areable to predict the oc-currence of snowslideswith great certainty. The avalanche, therefore, is a featureof mountain economy not less normal than the mountaintorrent. The most destructive avalanches occur in the first hoursof sunshine, just after a snow-storm. The flakes are thenso fine and smooth that almost any disturbance will startthem. A footstep or a gust of wind imparls motion to ahandful of snow, and it begins its descent. Gathering fresh. AVALANCHE BASIN, MONTANA Thr slopes are Inn sleep to permit the accumulationoj snow, and the latter, gathering within thebasin, lias formed the lake at the button! oi theclij]. WORK OF AVALANCHES AND GLACIERS 157 material as it advances, and increasing in velocity, it soonsweeps everything before it, carrying havoc and destructionperhaps into the region of cultivated fields, far beyond thefoot of the slope. Rocks crash right and left and the whirlof the wind carries eddies of snow a thousand feet or moreinto the air. i These, the poudreuses (powdery snow), are the most dreaded of allsnowslides. Damp snow does not shear and move readily; it is thelight, dry snow, that has little or no coherence, that is the distinctivefeature of this form of avalanche. When avalanches follow their customary tracks they areneither especially dangerous nor destructive, unless theyreach beyond their ordinary limits. But sometimes theytake place in localities previously fr
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